While many industrial gases are supplied to the site at which they are to be used in the form of so-called steel bottles, flasks or tanks under pressure and even in liquefied form, other gases are stored, transported and delivered under pressure and solubilized in an appropriate solvent.
For use, gas is drawn from the tank by expansion and hence desolubilization, the reduction in pressure resulting in the evolution of the dissolved gas therefrom and its discharge from the tank.
The tank can be discharged through one or more parallel-connected pressure reducers until gas is no longer evolved by the solution. Generally this point is reached some time after the gas supply from the tank is insufficient to cover the average flow requirements of the consumer although the tank was able to supply enough gas to cover the average flow requirement for the greater portion of the discharge. In other words, during the terminal phase of gas evolution, the volume of gas discharged from the tank is generally less than the mean flow requirement.
The rate and degree to which gas is evolved and discharged from the tank is dependent upon the tank pressure and hence the degree to which gas is ultimately released from its solubilized state is a function of the final pressure at the vessel and the temperature within the interior thereof.
With conventional systems, in which gas supply from the container may be cut off prematurely because of the inability of the container to supply the mean flow requirement, otherwise available gas remains in the tank and is not utilized.
For example, in the case of acetylene flasks or bottles, the acetylene can only be completely utilized (i.e. withdrawn from solution until the latter is simply saturated at ambient pressure with the gas) if the amount of gas withdrawn per unit time is low, i.e. the flow rate is well below the ordinary flow demand and the expansion is effected against an extremely low pressure (back pressure).
If one or the other of these conditions is not met, the bottle is found to contain more acetylene than the saturation quantity, i.e. gas which is present in the bottle or in the solution in an amount above the saturation level.
There are many reasons for this. For example, an excessively rapid flow rate during the terminal phase may prevent the evaporation heat, the desolubilization heat or any heat of adsorption from being communicated to the solvent or from being uniformly distributed in the solvent. If the back pressure is excessive, the bottle interior may ultimately be under a superatmospheric pressure which enables considerable gas to be stored in the solvent beyond the ambient saturation quantity.
A further reason for unsatisfactory emptying of the tank can be the pressure loss in the pressure-reducing device with dropping forepressure, defined for the purpose of the present distribution as the pressure of the gas ahead of the pressure reducer, which increases sharply so that there is frequently an extremely large pressure differential between the forepressure and the backpressure. For a given backpressure, this means an exceptionally high forepressure and thus a high pressure within the tank.